Internal combustion engines of motor vehicles such as, for instance, passenger cars or trucks, especially diesel engines, can make use of glow plugs to assist combustion in the internal combustion engine. Such glow plugs are also referred to as glow pencils. The glow plugs are actuated by a glow time control device using electric energy from the electrical system of the vehicle. Glow pencils are normally operated at a prescribed rated voltage of, for example, 5.5 volts. If the glow time control device is incorrectly connected during the assembly or servicing of the vehicle, voltages that can damage or destroy the glow time control device might be applied to the inputs and outputs of the glow time control device. A damaged or destroyed glow time control device, in turn, can damage a glow pencil that is connected to it.
In this context, German patent application DE 100 13 939 A1 discloses an electric circuit having reverse polarity protection. Reverse polarity protection is provided in the positive supply line of the circuit, and the reverse polarity protection has an N-channel MOSFET transistor with a Zener diode connected in parallel as well as a small-signal MOSFET transistor with a Zener diode connected in parallel. The drain terminals of the two transistors are provided for the feed via the positive supply line, whereas the gate terminal of the N-channel MOSFET transistor is provided for the feed via a power bridge circuit and is connected to the source terminal of the small-signal MOSFET transistor.
German patent application DE 10 2008 001 114 A1 discloses a control circuit for glow pencils that serves to protect against polarity reversal. A control unit comprises a first field-effect transistor that is interconnected with the glow plug in a shared current path. Moreover, there is a second field-effect transistor that is connected to the first transistor in the opposite direction so that the current flow in the current path is blocked in both directions. A switching unit is configured to activate the second transistor so that the current flow can be released in the direction of a current path corresponding to the prescribed polarity of the glow plug.
German patent application DE 100 48 184 A1 relates to a reverse-voltage protective circuit that serves to protect a control device that is supplied by a direct-voltage source against a reverse voltage in case of a polarity reversal. The reverse-voltage protective circuit comprises a MOSFET that is connected between the direct-voltage source and the control device, whereby the source terminal of the MOSFET is connected to the positive terminal of the direct-voltage source, and, by means of a driver that can be actuated by the control device, the gate terminal of the MOSFET can be impinged upon in such a manner that the MOSFET is switched on. The gate-source path can be protected against overvoltage by means of two Z-diodes that are series-connected in opposition and that are connected to each other via their anodes.
German patent application DE 198 177 90 A1 relates to a reverse polarity protection in which a semiconductor switch is connected in parallel to a reverse-polarity protection diode, and this semiconductor switch is switched off in case of a polarity reversal, whereas it is switched on during normal operation.
German patent application DE 198 40300 A1 relates to a reverse-polarity protection circuit for an electronic power output stage that can be actuated by a trigger circuit. When it comes to this reverse-polarity protection circuit, if the polarity of the supply voltage is incorrect, then power FETs are used in at least one supply terminal in order to interrupt the through-connection of the incorrectly applied supply voltage to the trigger circuit and to the power output stage.
German patent application DE 41 344 95 A1 relates to a control device for electric motors in motor vehicles which meets the requirements in terms of short-circuit strength, reverse polarity protection, load-dump protection, interlocks against malfunctions, etc. The reverse polarity protection is achieved by means of a transistor. If the motor is supposed to pick up current, then the transistor is switched on via two diodes as well as via a resistor. At this point in time, the transistor would be operated inversely. As a result, the parasitic diode (source-drain) of the transistor is greatly relieved since the current flows via the drain-source path. Only a greatly diminished power loss occurs.
Aside from a polarity reversal and an inadvertent connection of a terminal of the glow time control device to +12 V or to the ground, it can also happen that electrostatic discharges at the terminals of the glow time control device could cause the destruction of the latter. Moreover, especially in the case of a glow time control device that is suitable for actuating several glow pencils in a multi-cylinder engine, interactions can occur between the control outputs for the glow pencils. This can especially happen if several power switches, for instance, FETs, are actuated in the glow time control device by a shared control output of a control unit of the glow time control device.